Core B, the Clinical Core, will serve as a reservoir of well-characterized study subjects, both patients and controls, for behavioral and clinical research studies. As such, the Clinical Core will serve as the source of patients for the Chronic Fatigue Syndrome Clinical Research Center (CFS- CRS) Projects 3 and 5, as well as for projects using Developmental Funds. Core B has four major specific aims: l) to prospectively evaluate and follow a referral clinic-based cohort of patients will chronic fatigue and CFS; 2) to utilize this population as the basis for investigations on CFS; 3) to maintain databases on a variety of fatigued and control populations and to recruit new comparison groups to improve our understanding of CFS; and, 4) to examine the operating characteristics of clinical evaluation instruments already collected and to examine patient subgroups using available data. The results of a comprehensive evaluation, including information on demographic, medical, psychological, functional and social features on over 800 patients seen in a referral clinic are currently in our database. Ethnic/racial minorities comprise about 6% and women about 75% of the patients. Control groups available for comparison to CFS patients include healthy individuals and those with post-mononucleosis fatigue, major depression, multiple chemical sensitivities, rheumatoid arthritis, irritable bowel syndrome, fibromyalgia, and temporomandibular joint disorder. Comparisons between healthy and ill groups of subjects and CFS can help elucidate the predisposing factors, precipitants, predictors of outcome, and perpetuators of illness. Other questions amenable to study using the Clinical Core include the development of a battery of appropriate assessment measures for use in CFS and the evaluation of diagnostic test or marker.